SALINITY OF IRRIGATION WATER — SCOFIELD 285 



creases the difficulty of correct diagnosis. Impaired crop growth or 

 reduced yield may occur where the concentration of total salinity 

 is low, but where the trouble is due to the excessive concentration, 

 either absolute or relative, of some one constituent of that salinity. 

 The injurious effects of boron and of high ratios of sodium occurring 

 in solutions of low total salinity may be cited as examples. 



There are some crop plants that do exhibit recognizable symptoms 

 of injury from one or another of the salt constituents. These symp- 

 toms may be associated with impaired growth rate or they may occur, 

 in mild form, without measurable evidence of general growth de- 

 pression. As an example, it has been found that when the boron 

 content of the soil solution is approximately 3 parts per million the 

 leaves of lemon trees show a characteristic pattern of yellow color 

 in the tissue between the veins, with some marginal necrosis. This 

 symptom appears even when the degree of injury is so slight that 

 growth and fruiting are not measurably depressed. Other and no 

 less definite symptoms of boron injury are known in other plants, 

 and there are still other examples of abnormalities of color or form 

 in plants that are associated with nutritional derangements conse- 

 quent upon excessive or abnormal salinity conditions in the soil 

 solution. With some few exceptions, however, the adverse effects of 

 salinity constituents in the range of toxic concentrations are mani- 

 fested only by the depression of the rate of growth. 



RANGES OF PLANT TOLERANCE 



In attempting to set up standards of reference by which to trans- 

 late the data of water analysis into terms of the tolerance limits of 

 crop plants one encounters difficulties of several kinds. It has been 

 pointed out in an earlier paragraph that the plant has to deal with the 

 soil solution, and that there is no constant relationship between the 

 concentration of the irrigation water and of the soil solution. Because 

 of the practical difficulty of extracting a sample of the solution from 

 the soil in an undiluted condition we have fewer analyses of soil 

 solutions than we have of irrigation waters. Thus the temptation 

 is to compare the data of the analyses of the irrigation water with 

 the behavior of the crops on which the water is used. Such a com- 

 parison is hazardous not only on account of the variable relationship 

 between the irrigation water and the soil solution, but because the 

 effect produced on any given plant species by any given concentra- 

 tion of a solution constituent may be profoundly influenced by the 

 local climatic conditions. For example, a crop plant is more toler- 

 ant of salt in a humid coastal climate than in a dry interior climate. 



It should also be recognized that there are great differences among 

 the species of crop plants in respect to their tolerances to each of 



